Upgrading MacOS from El Capitan to Monterey issue

Those drives are two parts of what sounds like a split Fusion Drive.

The SSD part is to small on its own to upgrade and the HDD is to slow on its own.

What you need to do, is refuse the drives back into one Fusion Drive, then reinstall the macOS and upgrade.

see > How to fix a split Fusion Drive - Apple Support

Your SSD + HDD combination sounds suspiciously like something that used to be a 1 TB Fusion Drive – but got split (intentionally or otherwise). That SSD was never meant to be the whole startup drive (and even as part of a Fusion Drive, it's skimpy – 1 TB Fusion Drives originally included 128 GB SSDs!).

If you have a spare USB 3.0 hard drive or SSD, you could try

1. Formatting it in Disk Utility, as a Mac OS Extended (Journaled, Non-Case-Sensitive) volume.
2. Cloning your internal startup drive onto it, with the aid of a version of Carbon Copy Cloner or SuperDuper! that is old enough to be compatible with El Capitan.
3. Selecting the external disk as your startup disk (in System Preferences > Startup Disk, or by holding Option while booting to bring up the Startup Manager), and starting up from it.
4. Upgrading the external drive (which should have a lot more room than the internal SSD does) to Monterey.

Once the upgrade is complete, you could think about using a Monterey-compatible version of Carbon Copy Cloner or SuperDuper! to clone the external startup volume back to the internal SSD.

But if you install Monterey on an external USB 3.0 SSD, you might just decide that you'd rather keep that external drive as your main startup drive. It won't be as fast as the internal SSD would be, IF you could keep a reasonable amount of free space on that SSD.

If you re-fuse the halves of the Fusion Drive, as den.thed suggest, be sure to

• Make backups of the internal SSD and internal HDD, on external hard drives, before starting.
• Make sure that you have at least one bootable external drive (e.g., a clone backup of the internal SSD), before starting.

As the Apple Support article warns, re-joining the halves of the split Fusion Drive will "permanently delete all data stored on the drives that make up your Fusion Drive." You do not want to do that until you're certain that you can boot the machine, and restore your data, afterwards.

Given how small the SSD component of your Fusion Drive is, an external USB 3.0 SSD might be a better choice for your startup disk even if you rejoin the Fusion Drive. The internal SSD is faster than an external one would be, but there's so little of that "choice real estate" that you might overall, benefit from a 500 GB – 1 TB USB 3 SSD like one of these.

https://eshop.macsales.com/shop/owc-mercury-on-the-go-pro/usb-3.1

https://www.crucial.com/ssd/x9-pro/CT1000X9PROSSD9

Frylords wrote:

I'll see what kind of performance this drive offers once it's back up and running but I may take your advice in regards to the external SSD SoC.

The SSD component is on the small side – but at least, since your iMac is a 27" one, I believe that the hard drive should be a 3.5" 7200 rpm one.

Some of the 21.5" iMacs with 1 TB Fusion Drives had the unfortunate combination of a very small SSD (like that) paired with a 2.5" 5400 rpm hard drive. Those drives weren't as fast as the 3.5" 7200 rpm ones.